Conventionally, various types of capacitive occupant detection apparatus have been proposed (for example, see patent document 1: JP-2008-111809A corresponding to US2008/0100425A). As a related art, a capacitive occupant detection apparatus illustrated in FIGS. 40A and 40B will be described. In this capacitive occupant detection apparatus, a detection electrode 91 is placed in an inside of a seat 90 of a vehicle, and a guard electrode 92 is placed on a lower side of the detection electrode 91. A sinusoidal signal is applied to the detection electrode 91 to generate an electric field therearound, and a value of an electric current of the sinusoidal signal, which changes under influence of a conductive body and/or a dielectric body (e.g., a human body) located around the detection electrode 91, is detected. Then, according to this detection result, a detected capacitance or the like is computed, and it is determined whether an occupant on the seat 90 is an adult 93 or a child 94 on a child-restraint system (CRS).
According to the above related art, however, the electric field generated from the electrode 91 is directly coupled not only with a detection target (adult or child) 93, 94 but also with a conductive body 97 (vehicle body), which corresponds to a region surrounded by the dashed-line 95, 96. Thereby, the detected capacitance is increased by the amount corresponding to this direct coupling.
FIG. 41 illustrates capacitance components C1, C2 in each detected capacitance for different types of seated occupant. The capacitance C1 is the capacitance through the human body, and the capacitance C2 is the capacitance (direct coupling capacitance) increased by the direct coupling (i.e., direct coupling capacitance).
As shown in FIG. 41, since the child on the CRS covers a small area of the electrode 91, the child on the CRS causes larger the direct coupling capacitance C2 than the seated adult. Therefore, when the presence of a cushion between the adult and the seat and/or occupant's thick cloth(es) increases a distance between the electrode 91 and the body of the adult on the seat, the capacitance C1 becomes smaller even if the occupant is the adult. As a result, it becomes difficult to determine whether the occupant on the seat is the adult or the child on the CRS. This difficulty associated with the determination based on the direct coupling capacitance C2 can apply to not only a determination as to whether the occupant is the adult or the CRS but also a general determination as to a mounting state on a seat.
Patent Document 2 (JP-2010-286390A) describes a capacitive occupant detection apparatus in which a detection electrode (main electrode) is formed on a film board. The detection electrode is for detection of a capacitance to a human body serving as a detection target, and is connected to a control unit. The control unit has a function to measure the capacitance and perform an occupant determination. For reduction of cost or the number of part, the detection electrode is stretched thin and configured into a wire portion, so that the wire portion serves as a substitute for a harness that connects the detection electrode and the control unit.
The above connection manner, however, creates an unwanted capacitive coupling between the wire portion and the detection target or between the wire portion and the ground.